Basis weight control system for a paper making machine

ABSTRACT

IN ORDER TO PRODUCE A MORE ACCURATE FINAL BASIS WEIGHT AT THE REEL OR DRY END OF A PAPER MAKING MACHINE, THE BASIS WEIGHT VALVE IS ADJUSTED VIA A FEEDFORWARD CONTROL LOOP THAT PROVIDES A COMPUTED SIGNAL DERIVED FROM MEASURED HEAVY STOCK CONSISTENCY, FLOW AND FOURDRINIER WIRE SPEED. COMPENSATION, HOWEVER, IS MADE TO THE FEEDFORWARD LOOP BY A FEEDBACK CONTROL LOOP WHICH FURNISHES A SIGNAL COMPUTED FROM THE ACTUAL BASIS WEIGHT AT THE REEL. WHEN A GRADE CHANGE IS TO BE MADE, THE NEW SET POINT SIGNAL REPRESENTING THE DIFFERENT GRADE IS CAUSED TO TEMPORARILY BYPASS THE FEEDBACK CONTROL SO AS TO ALLOW THE FEEDFORWARD LOOP TO PROVIDE A RAPID ADJUSTMENT OF THE BASIS WEIGHT VALVE.

June 22, 1971 s. R. PERslK, JR., ErAL 3,586,601

BASIS WEIGHT CONTROL SYSTEM FOR APAPER MAKING MACHINE Filed Deo. 23, 1968 INN United States Patent O 3,586,601 BASIS WEIGHT CONTROL SYSTEM FOR A PAPER MAKING MACHINE Spencer R. Persik, Jr., Beloit, Wis., Marion A. Keyes IV,

South Beloit, Ill., and Douglas L. Bockstana, Silsbee,

Tex., assignors to Beloit Corporation, Beloit, Tex.

Filed Dec. 23, 1968, Ser. No. 786,009 Int. Cl. D21f 1/06 U.S. Cl. 162-252 7 Claims ABSTRACT OF THE DISCLOSURE In order to produce a more accurate final basis weight at the reel or dry end of a paper making machine, the basis weight valve is adjusted via a feedforward control loop that provides a computed signal derived from measured heavy stock consistency, flow and Fourdrinier Wire speed. Compensation, however, is made to the feedforward loop by a feedback control loop which furnishes a signal computed from the actual basis weight at the reel. When a grade change is to be made, the new set point signal representing the different grade is caused to temporarily bypass the feedback control so as to allow the feedforward loop to provide a rapid adjustment of the basis weight valve.

BACKGROUND OF THE INVENTION (l) Field of the invention This invention relates generally to paper making machines and pertains more particularly to a control system for producing a web of paper having a relatively uniform final basis weight, yet allowing a relatively fast change to a new basis weight when a different grade of paper is to be produced.

(2) Description of the prior art Owing to the process delay in a paper making machine, when an incorrect final basis weight is sensed at the dry end of the machine it is too late to make a correction at the Wet end of the machine. In other words, when the wet end is adjusted there is a time lag before the change reflects in the paper product at the dry end. By this time other process disturbances are apt to have arisen and the actual basis weight fluctuates, the excursions in many instances providing a subgrade paper or causing losses due to a higher fiber content than that specified for the grade being manufactured. In the past, operators have observed the extent of various disturbances, but since the manual correction for one disturbance will frequently accentuate the effect of another disturbance, human operators simply cannot make the proper analysis of the variations nor can they act fast enough when they have made a partially realistic determination.

SUMMARY OF THE INVENTION The present invention provides a control system that, in effect, can anticipate a needed adjustment in the flow of heavy stock and make the appropriate correction irnmediately so that a more uniform basis weight results in the web of paper as it is wound onto the reel.

Accordingly, the present invention has for one object the provision of a control system that substantially improves the quality of paper being made.

Another object is to provide a system that quickly reduces the adverse effects of disturbances and nonlinearities by providing a rapid response and an appropriate adjustment in the flow of heavy stock so that even slight process disturbances are corrected for before they become severe.

Another object of the invention is to permit changes 3,586,601 Patented June 22, 1971 ICC in the grade of paper to be smoothly made while the machine is still under control so that a saleable product is made during the transition. Also, the invention has for an advantage the drastic reduction in time required to change from one grade to another so that the amount of between grade paper that is produced is minimized.

A further object of the invention is to provide a control system that is simple and reliable, yet maintaining a desired nal basis weight.

A still further object is to provide a control system of the envisaged character that lends itself to being made in modular form, thus facilitating its maintenance and also allowing the system to be expanded upon as circumstances dictate.

Yet another object of the invention is to provide a system of control that can be incorporated into existing paper making machinery without difficulty.

BRIEF DESCRIPTION OF THE DRAWING The single ligure illustrating the invention is a combined block and schematic diagram depicting sections of a conventional paper making machine with our control system added thereto.

DESCRIPTION OF THE PREFERRED EMBODIMENT The paper making machine denoted generally by the reference numeral 10 is of conventional construction. However, it is necessary to illustrate salient sections of such a machine in order to illustrate certain locations where operational data is sensed.

Accordingly, the paper making machine 10 includes a headbox 12 which spreads the mixed stock onto a web forming means which is illustrated as a Fourdrinier wire section 14 having a breast roll 16 and a drive roll 18. The whitewater is collected in a wire pit 20. A stuff box 22 contains the heavy fiber stock which is fed through a basis weight valve 24 and then combined with the whitewater from the pit 20. A fan pump 26 elevates the mixed stock into the headbox 12. The web of paper indicated by the reference numeral 28, while still wet, is directed from the wire section 14 through a press section 30, a drying section 32 and in its dried state it is wound onto a reel 34.

Referring now to the system for controlling the basis weight at the reel 34, it will be observed from the drawing that the system has been denoted in its entirety by the reference numeral 50. Depending upon the grade of paper and hence the particular basis weight that is desired at the reel end of the machine 10, an appropriate set point is provided at 52. When a different grade of paper is to be produced, a new set point will be used. The particular set point may be entered manually or the proper value taken from a magnetic disc, drum or core storage. Cooperating in the smooth change from one set point to another is a delay means 54, the need for which will become more apparent as the description progresses.

In the illustrated situation, the desired set point signal passes through the delay means after a suitable length of times has elapsed to a comparator 56 having a pair of input terminals 56a, 56h and an output terminal 56e. The delay means 54 is actually connected to the input terminal 56a and the input terminal 56]; is connected to a computer that is identified in the drawing as a feedback basis weight circuit 58. The network circuit 58 has a number of input lines 60, 62 and 64. The input line 60 delivers information relating to basis weight range, the input line 62 data concerned with moisture obtained from an appropriate transducer (not show-n) and the input line 64 data sensed from a conventional beta gauge 66. For the sake of simplicity, it will be assumed that the gauge 66 is of the integrating type and that the signal arriving via the line 64 represents the Valid average. Of

course, it will be understood that the gauge `66 could forward instantaneous information and the 'valid average could be computed after a complete scan by the gauge 66 when of the non-integrating type. All that need be provided is a signal that is delivered to the feedback circuit 58 |When it is processed along with other input data, namely, the inputs entered on lines 60 and 64, to provide an appropriate feedback signal that is indicative of the actual basis weight in the web of paper 28 as it is wound on the reel 34 after the drying section 32 has reduced its moisture to only several percent.

The feedback signal from the circuit 58 is compared by the comparator 56 with the signal representing the desired set point that is forwarded over the line 52 and any difference or error is applied by way of the output terminal 56a to a feedback basis weight controller 68. The feedback controller `68 performs an integrating function, the integral signal at the output side thereof being applied to another comparator 70 by way of its input terminal 70a. The comparator 70 has a second input terminal 70'b which will be referred to in greater detail hereinafter and has an output terminal 70e which is attached to the input terminal of a ratio set point controller 72 which is also of the integrating type. The output from the ratio set point controller 72 represents an adjusted or modified set point signal 'which is applied to a comparator 74, more specifically, its input terminal 74a. The comparator 74 has a second input terminal 74b and an output terminal 74e which is connected to a pulse duration basis Weight controller 76. The controller 76 is utilized to adjust or position the valve 24 so as to vary the rate of heavy stock iiow to the pump 26 and subsequently to the wire or web forming section 14 via the headbox 12. It is important to appreciate that the controller 76, in contradistinction to the controllers 68 and 72, performs an actuating function, specifically, the positioning of the basis Weight valve 24.

At this time, attention is called to the need for signals representative of certain operating data made use of in the control system S0. In this regard, a consistency transducer 80 is located so as to measure the consistency of the heavy stock and provide a signal in accordance therewith. More specifically, the transducer 80 measures the consistency of the heavy stock as it is delivered from the stuff box 22. Also, a flowmeter transducer 82 provides a signal in accordance with the rate of flo-W of the heavy stock. Still further, a wire speed tachometer 84 is connected to the breast roll 16 of the Wire section 14 so as to provide a signal indicative of the speed at which the fwire section is actually moving.

Playing an important role in the practicing of our invention is a feedforward basis weight circuit 86 that provides a computed sig-nal representative of the bone dry basis Weight. The mathematical model is:

Where:

B=Bone dry basis weight C=Heavy stock consistency measurement F=Heavy stock ow (gpm.) V=Fourdrinier lwire speed K1=One proportionality constant, and K2=A second proportionality constant The values for K1 and K2 may be obtained empirically or with a special program which applies a least mean squares fit to the variables appearing above.

With the foregoing in mind, it is believed that the function of the feedforward basis weight circuit 86 will now be better understood as far as the computations expected of it. In this regard, the signals representative of rough stock consistency (C i-n the above equation) and rough stock rate of flow (F in the above equation) are fed to a multiplier 88 having first and second input terminals 88a, 8817 and an output terminal 88e. The output terminal 88e` is connected directly to a multiplier 90 having a-n input terminal 90a which is connected to the output terminal `88a` and a second input terminal 90b which introduces into the multiplier 90 a signal representative of the constant K1, the means for providing this signal being schematically portrayed by the potentiometer 92. The product signal at the output terminal 90C of the multiplier is forwarded to a divider 94, more specifically its input terminal 94a, whereas its second input terminal 94b is connected to the tachometer 84 so as to cause the product signal impressed on the input terminal 94a to be divided by the signal (V in said equation) obtained from the tachometer 84. The quotient signal appearing at the output 94a of the divider `94 is then delivered to an adder 96, more specifically its input terminal labeled 96a, and the second input terminal 96b of the adder 96 is attached to a potentiometer 98 that schematically causes a signal in accordance with the constant K2 to be introduced into the feedforward circuit 86.

It is contemplated that any number of grade changes can be made which will require different basis Weights at the reel 34 and that the set point applied at 52 is simply representative of the desired Value, whether set manually or taken from suitable storage. To shift the machine 10 quickly toward the new basis Weight, a bypass line 108 is connected intermediate the set point line 52 and the delay means 54. The line 10r8 leads to an input line .110a of a ratio input device 110 so that the new set point will be divided by whatever adjusted or modified set point signal is already present at the input line 110b, this being the same signal as applied to the input terminal 74a of the comparator 74. -It is the output line 110C that carries the signal in accordance with the developed ratio which is impressed on the earliermentioned input terminal 70b of the comparator 70. Hence, whenever there is a change in the set point value, in order to produce a desired change in grade o-f paper at the reel 34, this will quickly change the ratio of the desired set point signal applied to the input line 110a with respect to the already existing modified set point signal on the input line 110b. The sizable signal then furnished by the ratio input device |110 over its output line 110el is impressed on the input terminal 70b of the comparator 70 with the consequence that a large difference signal appears at the output terminal 70C of the comparator due to the fact that the signal at the input terminal 70a has not had a chance to change. Consequently, whatever control the feedback circuit 58 is exercising is effectively bypassed by virtue of the action just described.

OPERATION Although the manner in which our system operates should be understood from the foregoing description, nonetheless a brief operational sequence should be of help in providing a full appreciation of the benefits to be obtained. If a process disturbance is occasioned by a change in heavy stock consistency, a change in the rate of iiow of heavy stock or in the speed of the Fourdrinier Wire section 14, such as disturbance will ultimately affect the basis weight of the paper web 28 as it leaves the machine 10 to be Wound on the reel 34. If only a sensing of the final basis weight is resorted to, then the feedback circuit y58, although capable of making a correction, will cause the correction at the web end of the machine to occur but by the time that any such correction at the wet end has had time to manifest itself at the dry end and be sensed by the gauge 66, a further disturbance may well have been introduced into the paper making process. Therefore, the correction would be entirely too late and in many instances could actually cause an overcorrection or undercorrection as far as the final basis weight is concerned which would make the correction so adverse as to have made it better if no correction had even been attempted. This has been particularly true in prior art systems where a human operator must note the existence, direction and magnitude of the disturbing variable.

However, where a particular desired set point is supplied by the line 52, there will be a comparison of this signal at the comparator 56. Assuming for the moment that the feedback circuit 58 is furnishing a signal that is of substantially the same order as the set point at 52, there |will be only a small error or difference signal at the output terminal 56C of the comparator `56. This signal, when integrated by thecontroller 68, will still be small and will result in a correspondingly small error signal from the controller 72.

Since the modified set point signal at the intput 74a of the comparator 74 is compared with the feedforward signal provided by the circuit -86, if these two signals are approximately equal, then there will be only a small difference signal at the output terminal 74e of the comparator 74. This signifies that the entire paper making machine is operating in a stable condition and that there is little or no disturbance occurring. Therefore, only the small corrective or control signal from the comparator 74 will be utilized to operate the pulse duration basis weight controller 76. Consequently, only a small adjustment of the basis weight Valve 24 will take place, for a large adjustment is not needed under this set of assumed conditions. Stated somewhat dilferently,jany disturbances that would be affecting the paper` making process are small and hence any corrective effort that is needed is very minimal because the correct basis weight at the reel 34 is being maintained.

Through the agency of the feedforward basis Weight circuit '86, however, any disturbance contributed by a change is consistency of the heavy stock,z a change in its rate of flow or a change in the speed of the wire section 14, any of vwhich would precipitate the corerction outlined in the above paragraph, will be processed in the circuit 86 to furnish a change feedforward signal that is impressed on the input terminal 74b of the comparator 74. The computation that occurs within the circuit 86 is quite rapid and thus any change sensed by the gauge 66 which starts the corrective action is literally anticipated by the rapid response provided by the feedforward circuit and will initiate an immediate correction in the proper direction.

More specifically, with respect to the operation of the feedforward basis weight circuit 86, it should be observed that any change detected by either the consistency transducer l80 or the fiowmeter transducer 82 when multiplied by the other at the multiplier 88 will produce a product signal at the output terminal 88C which Will in turn be multiplied at the multiplier 90 by the constant signal delivered from the potentiometer 92 to the multiplier 90 via its input terminal 90b. This, in and of itself, will result in a different quotient signal at the output terminal 94e of the divider 94. On the other hand, if there is also taking place a change in the wire speed, such a change will be sensed by the tachometer 84 with the result that the divisor signal applied to the input terminal 92b will cause a change in the feedforward signal.

lRecapitulating for a moment, if the gauge or transducer 66 is sensing a different actual basis weight from that commanded by the signal at 52, there will be a signal fed back from the circuit 58 which will also be different from that ordered by the set point. Because of this situation, there will be a difference or error signal appearing at the output terminal 56e of the comparator 56. After integration by the controllers 68, 72, there will be a signal at the input terminal '74a of the comparator 74` that will compensate ther feedforward loop, since its signal is being applied to the other input terminal 74b of the comparator 74. lOf course, it will be appreciated that the feedforward circuit 86 is constantly receiving operational data from the transducers 80, 82 and 84 so if any change in the process variables is detected by any of these three sensors, such a change will be quickly reected in the feedforward signal which is applied to the input terminal 74b. The corrective or control signal at the output terminal 74e, as determined by both the feedback and feedforward signals, will cause the basis weight controller 76 to adjust or reposition the basis weight valve 2.4 so that the rate of heavy stock flow therethrough is either increased ordecreased and in this way more uniformly control the final or actual basis weight at the reel 34.

It should be explained at this stage that in order to avoid double correction as far as the control afforded by the feedback circuitry 58, the feedbacksignal should be computed yby using the old value for the previous two scans made by the gauge 66. This will allow time 4for the feedforward correction (more precisely the operating results therefrom) to reach the dry end of the machine 10 where the beta gauge 66 is located. The feedback controller 68 actually computes a new set point ratio for every scan period. The ratio loop device 110, operating every second, for instance, exponentially slews the control signal provided by the output terminal 74e to its new value as dictated by Whatever ratio is required. The important thing to keep in mind is that the basis weight valve Z4 is manipulated in a direction such that the predicted basis weight equals the modified set point signal as computed by the ratio loop.

From what has been hereinbefore said, it should be understood that the feedforward action is relatively rapid, whereas the feedback action is relatively slow. Consequently, when a new grade of paper is to be manufactured, a new desired set point must be furnished at 52. The feedback action is simply bypassed by directing the new set point signal to the ratio input device and it quickly produces a new ratio signal that is compared at the comparator 70. This quickly causes the comparator 74 to produce a new control or corrective signal that repositions the valve 24 rapidly so that the ow of heavy stock is yimmediately changed. Such change will be reflected much earlier in the final basis weight at the reel 34, keeping the basis weight value quite uniform. It is the function of the delay means 54 to introduce a time lag that does not immediately cause a difference signal to be produced at the output terminal 56C of the comparator 56 and hence the signal at the input terminal 70a of the comparator 70 is not changed, thereby allowing a greater difference ysignal to be provided at the output terminal 70e` of the comparator 70 with the consequence that a different modified or adjusted set point signal appears at the input terminal 74a of the cornparator 74. Such action provides the corrective signal utilized by the pulse duration basis weight controller 76 in repositioning the valve 24. i

Having presented the foregoing information, it should be readily apparent that a paper making machine 10 when equipped with our control system 50` results in a more uniform final basis weight and also facilitates any change from one grade of paper to another.

We claim:

1. In a machine -for making a continuous web of paper, said machine having a web forming section, a headbox and a basis weight valve for varying the flow to said headbox of heavy stock subsequently delivered to said web forming section via said headbox, a system for controlling the final basis weight of said web comprising means providing a set point signal indicative of a desired basis weight, means providing a feedback signal indicative of the actual final basis weight, first means comparing said desired set point signal with said feedback signal to provide a modified set point signal representing any difference present between said signals, means providing a feedforward basis 'weight signal derived from the consistency and the rate of ow of heavy stock through said valve and the speed of said web forming section, second means for comparing said feedforward signal with said modified signal to produce a corrective signal representing any difference between said feedforward signal and said modified signal, and means responsive to said corrective signal for adjusting the position of said valve to increase or deci-ease the flow rate therethrough to produce any needed change in the final basis weight of said web.

2. The machine and control system of claim 1 wherein said feedforward basis weight means includes a transducer for deriving a first signal representative of the consistency of the said heavy stock, a transducer rfor deriving a second signal representative of the rate of heavy stock flow, a transducer for deriving a third signal representative of the speed of the web forming section, said feedforward means also including a multiplier for multiplying said first and second signals to provide a product signal and a divider for dividing said product signal by said third signal to provide said feedforward signal.

3. The machine and control system of claim 2 including means ttor introducing first and second predetermined constant signals to said feedforward means, said feedforward means including an additional multiplier for multiplying said first constant signal by said product signal to provide a modified product signal so that modified product signal is divided by said third signal to provide a quotient signal, and an adder for summing the quotient signal from said divider with said second constant signal to provide said feedforward signal.

4. The machine and control system of claim 1 including first controller means connected between said first comparing means and said second comparing means so that said modified set point signal is indicative of the integral of any difference signal derived from said first comparing means.

5. The machine and control system of claim 4 including delay means interposed between said desired set point means and said first comparing means so that a change from one desired set point signal value to an other desired set point signal value produces a time lag with respect to said first comparing means as contrasted with said second comparing means.

6. The machine and control system of claim 5 including bypass means connected between said desired set point means and said second comparing means `for applying said desired set point signal directly to said second comparing means so that said feedback signal is then compared with the desired set point signal.

7. The machine and control system of claim 6 including second controller means having its output connected to said second comparing means, a third comparing means having first and second inputs and an output, said first input being connected from said first controller means and said output of said second comparing means being connected to said second controller means, said bypass means including a ratio device having a rst input connected from said desired set point means and a second input connected from the output of said second controller means, the output of said ratio device applying a ratio signal to the second input of said third comparing means.

References Cited UNITED STATES PATENTS 1/1970 Hart et a1 162-252X OTHER REFERENCES s. LEON BAsHoRE, Primary Examiner A. DANDREA, J R., Assistant Examiner U.S. Cl. X.R. 162-198, 263 

